Electroacoustic transducer held together by thermoplastic clamping ring

ABSTRACT

A moving coil electroacoustic transducer is held together by a thermoplastic clamping ring. The ring has an integral flange which provides spaces between the magnetic system and the diaphragm. Acoustic damping is provided by a nonmagnetic grid located behind a voice coil and coupled acoustically to the space behind the diaphragm.

iJnited States aterit lnventors Appl. No.

Filed Patented Assignee Priority Leslie Eugene Basillliyrnolte-lfiradshnw Edgware;

Norman William Tester, London, both of England .lluly 30, 1969 Oct. 119,19711 International Standard Electric Corporation New Yorlr, NY.

Aug. 8, 1968 Great Britain ELECTROACOUSTIC TRANSDUCER HELD ReferencesCited UNITED STATES PATENTS 2/1952 Skelton 29/594 3/1932 Thuras...l79/115.5 6/1933 Giles 179/1 15.5 8/1942 Proctor 179/1 15.5 1/1949Gileman 1. 179/179 9/1950 Zimmerman 179/1 15.5

FOREIGN PATENTS 8/1963 Germany 179/115.5 PC

1962 Germany 179/1 15.5

1966 Switzerland 179/1 15.5

Primary Examiner-llfiathleen l-l. Cllaffy Assistant Examiner-Thomas L.Kundert Att0rneysC. Cornell Remsen, Jr.,, Walter J. Baum, Percy P.Lantzy, .1. Warren Whitesel, Delbert P. Warner and James ABSTRACT: Amoving coil electroacoustic transducer is held 'rocnrnnn m1rnnniviormsric 13111111 11511; Radc nine 7 Claims, d Drawing Figs.

11.5.1131 .179/1155 R int. Cl HMr 9/02 ll' ield of Search...- 179/115.5,

115.5 ES, 178, 179, 11s; 1s1/31;175/115.5 PC;

together by a thermoplastic clamping ring. The ring has an in tegralflange which provides spaces between the magnetic system and thediaphragm. Acoustic damping is provided by a .nonmagnetic grid locatedbehind a voice coil and coupled acoustically to the space behind thediaphragm.

ELEQTlllUACOUSTlC TRANSDUCER HELD TOGETHER WI THERMOPLASTTC ClLAMlPTNGMING This invention relates to electroacoustic transducers.

According to one aspect of the invention there is provided anelectroacoustic transducer, in which the front cover, back cover anddiaphragm are held together by a cylindrical thermoplastic clamping ringwhich has its two ends heat sealed over the front and back coversrespectively.

According to this same aspect of the invention there is further provideda moving coil electroacoustic transducer, in which the magneticstructure consists of a solid cylindrical permanent magnet topped by aninner pole piece of soft magnetic material and encased concentrically bya cup-shaped yoke of soft magnetic material, in which a diaphragmcarries a voice coil which rests in the annular airgap between the innerpole piece and the yoke, in which part of the yoke forms the back coverof the transducer, and in which the back cover, diaphragm and frontcover are held together by a cylindrical clamping ring, with thediaphragm being located on an inwardly extending flange of the clampingring and the two ends of the clamping ring being formed over the backand front covers of the transducer respectively.

According to another aspect of the invention there is also provided amoving coil clectroacoustic transducer, in which the magnetic structureconsists of a solid cylindrical permanent magnet topped by an inner polepiece of soft magnetic material and encased concentrically by acup-shaped yoke of soft magnetic material, in which a diaphragm carriesa voice coil which rests in the annular airgap between the inner polepiece and the yoke, in which part of the yoke forms the back cover ofthe transducer, and in which sound equalization is achieved by a dampinggrid behind the moving coil having one or more holes leading to anotherwise closed space between the permanent magnet and the yoke, and byone or more holes in the front cover of the transducer which also servefor the ingress or egress of sound.

A moving coil, electroacoustic transducer according to the inventionwill now be described with reference to the accompanying drawings, inwhich:

FlG. 1 shows a cross section view, along the line A-A of FIG. 2 of amoving coil transducer according to the invention,

FlG. 2 shows a plan view of the moving coil transducer shown in FIG. ll,

FIG. 3 illustrates modification to the transducer shown in FIGS. 1 and 2with regard to securing the permanent magnet of the assembly, and

FIG. A shows a section view of the clamping ring before it is used tohold together the assembly of parts of the transducer shown in F108. 1and 2, or FIGS. 1 and 2 as modified by FIG. 3.

Referring now to the drawings, a solid cylindrical permanent magnet 111of columnar material, such as Ticonal GX, is topped by an inner discpole piece 11 of mild steel of greater diameter than the permanentmagnet 10. A cup-shaped yoke of mild steel concentrically encases thepermanent magnet the yoke being made up of a generally frustoconical pot12, which forms the back cover of the transducer, and a flat annularpole piece 113.

A plastic diaphragm has a dome-shaped inner portion M and a flat rim 15.The diaphragm carries a voice coil 16 which rests in the annular airgapbetween the inner pole piece 111 and the outer pole piece 113. A dampinggrip 17, of nonmagnetic metal, is in the form of an annular troughbehind the voice-coil 1b with its inner rim attached to the inner polepiece 11 and it outer rim attached to the outer pole piece 13 Thedamping grid 17, has one or more holes 171 leading to an otherwiseclosed space between the permanent magnet and the yoke.

The magnet 10 is secured by an adhesive to the pot 12. FIG. 3 shows amodification to the shape of the pct 12 in which the magnet is securedby making it a press fit. The larger end of the pot 12 has an outwardlyextending flat lip 121 on which rests the flat annular outer pole piece113. The pot 12 and the outer pole piece 13 are held together by acylindrical clamping ring 18 (see also FIG. 1) ofthermoplastic-insulating material,

being clamped between one side of an inwardly extending flange 1M of theclamping ring 1d and one end 1M of the clamping ring 1% which is heatsealed over the flat lip 121.

The flat rim 15 of the diaphragm is located on the outer side of theflange 1M. A fiat annular washer 19, e.g., of thermosetting plasticmaterial, rests on the flat rim 15 of the diaphragm and a flat rim 201of the front cover 20, e.g., of mild steel, rests on the washer 19. Theother end 11131 of the clamping ring 1% is heat sealed over the flat rim2111, thus clamping the diaphragm between the flange 1M and the washer19 and holding the front cover 20 and diaphragm to the magnetic assemblyof the transducer.

The clamping ring 13 has an outwardly extending rib WA into which thetwo terminals 21, e.g., of brass, are secured. Each electrical lead 161of the voice coil 16 extends along a groove in the flange 1181 andthrough a hole or slot in the side of the clamping ring 18 and is laidover the top of the respective terminal 21 by locating it in a groove1185 in the outer lip of the rib 11M. The leads 161 are soldered orwelded to the terminals 21 and are protected by sealing material 22 inthe upper surface of the rib 111 1.

A preferred method of manufacture of the diaphragm is to form it betweenheated male and female tools, followed by suitable cooling. The coil ismade of solderable enameled copper wire coated with an adhesive and isself supporting, requiring no former.

Ticonal GX type-material is chosen for the permanent magnet working atits (Bl-1) maximum point, where B is the flux density in the permanentmagnet and H is the magnetizing force in the permanent magnet. Thismaterial is chosen because it has a low value of A/(BH),,,,,,, where Ais the density of the material, so as to achieve a low weight of magnetto provide a given flux density in a given airgap.

The antiresonant type of frequency response equalization is chosen forthe transducer. This is achieved by the hole or holes 171 in the dampinggrid 17 behind the moving coil. The frequency response is also modifiedby one or more holes 202 in the front plate 20. The holes 171 and 202may be damped by covering them with fabric or by making them in the fonnof louvres. The hole or holes 201 also serve for the ingress or egressof sound.

The magnetic efficiency factor G Z where G, the force factor, is definedas force per unit current or flux density in the airgap X conductorlength and Z is the AC impedance of the winding, is optimized to achievea flux density of the order of 7,500 lines per square cm. it is foundthat high sensitivity occurs for a magnet in the region of 5 to 10 gramsand for an airgap 0.1 cm. long.

The weight of the complete transducer capsule is of the order of 18grams, and the overall diameter and overall depth of the transducercapsule are approximately one inch and three-quarters of an inchrespectively.

The above-described transducer is of small size and lightweight and canbe used both as the transmitter and as the receiver of a telephone set.Modifications to the frequency response that may be required for thesetwo different applications may be obtained by adjusting the acousticmass and resistance of the holes in the associated mouthpiece or earcapand/or by adjusting the volume between the transducer front plate andthe earcap or mouthpiece. if a cut in sensitivity at low frequencies isrequired this can be achieved by making one or more holes in thediaphragm.

As a receiver it has the advantages over the electromagnetic transducerof the rocking armature type at present used in telephones etc., ofhaving a nearly constant impedance over the frequency band and of havingno narrow airgaps which can cause failure of the transducer in serviceif they become blocked with dirt or if the armature poles.

The transducer has lower distortion than the rocking-armature type.

As a transmitter the transducer has the advantages of lower distortionthan the carbon transmitter and does not suffer from burning noise.Because of its lower sensitivity than the carbon transmitter it must beused in conjunction with a transistor amplifier in the telephone set.However, the carbon transmitter, when subjected to much use, e.g., inpublic phone kiosks, and, more particularly when used in dirty locationssuch as railway stations and coal mines, becomes contaminated andreplacement is frequent. The moving coil transducer (and amplifier) doesnot, of course, suffer from this defeet and, being inherently morereliable, ensures fewer replacements.

From a manufacturing point of view, the production spread of performanceof this type of transducer is less than that of the moving iron type.

We claim:

I. An electroacoustic transducer, comprising a front cover, a back coverand a diaphragm, means holding said front cover, said back cover andsaid diaphragm together including a cylindrical thermoplastic-clampingring, in which said thermoplastic-clamping ring includes two ends whichare heat sealed over the front and back covers respectively, thediaphragm of the assembly is located on an inwardly extending flange ofthe clamping ring, the two ends of the clamping ring are formed over theback and front covers of the assembly respectively, and the clampingring is of thermoplastic-insulating material and has an outwardlyextending rib in which the terminals of the transducer are secured.

2. A moving coil electroacoustic transducer comprising a front cover, amagnetic structure, a diaphragm and a back cover, in which the frontcover is formed of a flat piece of mild steel having a flat outer rim,the magnetic structure consists of a solid cylindrical permanent magnettopped by an inner pole piece of soft magnetic material and encasedconcentrically by a cup-shaped yoke of soft magnetic material, thediaphragm carries a voice coil which rests in an annular airgap betweenthe inner pole piece and the yoke, the back cover is formed using partof the yoke, and the back cover, diaphragm, and the front cover are heldtogether by a cylindrical clamping ring, the clamping ring is formed ofthermoplastic-insulating material, the diaphragm is located on aninwardly extending flange integral to the clamping ring and the two endsof the clamping ring are formed over the back and front covers of thetransducer respectively.

3. A transducer as claimed in claim 2, in which the transducerincludesterminals and electrical leads from the moving coil, and the clampingring has an outwardly extending rib in which the terminals of thetransducer are secured, and in which the electrical leads from themoving coil extend through holes in the clamping ring to said terminals.

4. A transducer as claimed in claim 3, in which antiresonantequalization is provided by modifying frequency response with a dampinggrid behind the moving coil having one or more holes leading to anotherwise closed space between the permanent magnet and the yoke, inwhich the frequency response is also modified by one or more holes inthe front cover of the transducer which also serve for the ingress oregress of sound.

5. A transducer as claimed in claim 4, in which the inner pole piece isa disc of greater diameter than the permanent magnet, and in which thedamping grid is in the form of an annular trough with its inner andouter rims attached to the inner pole piece and the yoke respectively.

6. A transducer as claimed in claim 3, in which the part of the yokeforming the back cover of the transducer is generally frustoconical withits smaller end fitting the back end of the permanent magnet.

7. A transducer as claimed in claim 2, in which one end of the clampingring is sealed over one side of a flat rim of the front cover, in whichthe other side of the flat rim of the front cover rests on one side of aflat annular washer, in which a flat rim of the diaphragm is heldbetween the other side of the flat annular washer and one side of thethe inwardly extending flange of the clamping ring, in which the yokeconsists of a pot forming the back cover of the transducer and having anoutwardly extending flat lip with a flat annular outer pole pieceresting on said flat lip, and in which said pot and said outer polepiece are held together between the other side of the inwardly extendingflange of the clamping ring and the other end of the clamping ring whichis sealed over the flat lip of said pot.

1. An electroacoustic transducer, comprising a front cover, a back coverand a diaphragm, means holding said front cover, said back cover andsaid diaphragm together including a cylindrical thermoplastic-clampingring, in which said thermoplastic-clamping ring includes two ends whichare heat sealed over the front and back covers respectively, thediaphragm of the assembly is located on an inwardly extending flange ofthe clamping ring, the two ends of the clamping ring are formed over theback and front covers of the assembly respectively, and the clampingring is of thermoplastic-insulating material and has an outwardlyextending rib in which the terminals of the transducer are secured.
 2. Amoving coil electroacoustic transducer comprising a front cover, amagnetic structure, a diaphragm and a back cover, in which the frontcover is formed of a flat piece of mild steel having a flat outer rim,the magnetic structure consists of a solid cylindrical permanent magnettopped by an inner pole piece of soft magnetic material and encasedconcentrically by a cup-shaped yoke of soft magnetic material, thediaphragm carries a voice coil which rests in an annular airgap betweenthe inner pole piece and the yoke, the back cover is formed using partof the yoke, and the back cover, diaphragm, and the front cover are heldtogether by a cylindrical clamping ring, the clamping ring is formed ofthermoplastic-insulating material, the diaphragm is located on aninwardly extending flange integral to the clamping ring and the two endsof the clamping ring are formed over the back and front covers of thetransducer respectively.
 3. A transducer as claimed in claim 2, in whichthe transducer includes terminals and elEctrical leads from the movingcoil, and the clamping ring has an outwardly extending rib in which theterminals of the transducer are secured, and in which the electricalleads from the moving coil extend through holes in the clamping ring tosaid terminals.
 4. A transducer as claimed in claim 3, in whichantiresonant equalization is provided by modifying frequency responsewith a damping grid behind the moving coil having one or more holesleading to an otherwise closed space between the permanent magnet andthe yoke, in which the frequency response is also modified by one ormore holes in the front cover of the transducer which also serve for theingress or egress of sound.
 5. A transducer as claimed in claim 4, inwhich the inner pole piece is a disc of greater diameter than thepermanent magnet, and in which the damping grid is in the form of anannular trough with its inner and outer rims attached to the inner polepiece and the yoke respectively.
 6. A transducer as claimed in claim 3,in which the part of the yoke forming the back cover of the transduceris generally frustoconical with its smaller end fitting the back end ofthe permanent magnet.
 7. A transducer as claimed in claim 2, in whichone end of the clamping ring is sealed over one side of a flat rim ofthe front cover, in which the other side of the flat rim of the frontcover rests on one side of a flat annular washer, in which a flat rim ofthe diaphragm is held between the other side of the flat annular washerand one side of the the inwardly extending flange of the clamping ring,in which the yoke consists of a pot forming the back cover of thetransducer and having an outwardly extending flat lip with a flatannular outer pole piece resting on said flat lip, and in which said potand said outer pole piece are held together between the other side ofthe inwardly extending flange of the clamping ring and the other end ofthe clamping ring which is sealed over the flat lip of said pot.